1. Field of the Invention
This invention relates to a magnetic carrier contained in a developer used in electrophotography and electrostatic recording, and a two-component developer having this magnetic carrier and a toner.
2. Description of the Related Art
Developing systems of electrophotography include a one-component developing system, which makes use of a toner only, and a two-component developing system, which makes use of a toner and a magnetic carrier in blend. In the two-component developing system, a magnetic carrier that is a charge-providing member and a toner are blended and used as a two-component developer. The two-component developer provides so many opportunities of contact between the charge-providing member magnetic carrier and the toner as to promise stable triboelectric charge characteristics, and is admitted to be advantageous to the maintenance of high image quality. Also, the magnetic carrier supplies the toner to developing zones, and its supply can be large and is readily controllable. Accordingly, it is often used in, in particular, high-speed machines. In order to keep electrophotographic developing systems applied to print on-demand (POD), which attracts notice in recent years, it is important for the systems to be adaptable to three basic factors, i.e., high speed, high image quality and low running cost. Further, taking account of the application of two-component developers to the POD market, two-component developers are desired which enable reproduction of images which are free of any image defects in images reproduced in print, high in quality level and free of any variations in color tone and density over a long period of time.
Japanese Patent Laid-open Application No. H04-93954 discloses a proposal of a magnetic carrier having surface unevenness coming from fine crystal particles of the surfaces of spherical ferrite particles, in order to keep any image density variations from occurring because of long-term service. This is a magnetic carrier the cores of which have been so coated with a resin that their hills (or protrusions) may come bare to the surfaces, and which can be small in environmental dependency and be small in image density variations even in long-term service. However, this magnetic carrier has apparent density which is so as high as 2.66 g/cm3 that the carrier may heavily be stressed in a high-speed development process which is adaptable to the POD. Also, because of its coat resin layers designed to be small in thickness, it has come about that the magnetic carrier becomes low in electrical resistance because of scrape-off of the coat resin. Still also, the coat resin binds directly with spherical ferrite cores, and hence the coat resin and the cores may have an insufficient adherence between them, so that the coat resin may come to come off to make the magnetic carrier have a low electrical resistance. In such a case, especially when the two-component developer is left to stand for a long term in a high-temperature and high-humidity environment after long-term service, it may cause fog or great image density variations. In addition, a phenomenon that electric charges are injected from a developing sleeve into an electrostatic latent image bearing member through the magnetic carrier may come about to disturb latent images on the electrostatic latent image bearing member to make halftone areas coarse.
Accordingly, a magnetic material dispersed resin carrier is proposed, in which a magnetic material has been dispersed in a resin in order to more make the carrier lower in specific gravity and lower in magnetic force. Japanese Patent Laid-open Application No. H08-160671 discloses a proposal of a magnetic material dispersed resin carrier which is high in electrical resistance and low in magnetic force. Such a carrier can achieve improvement in sufficiently high image quality and high minuteness and in higher durability as it has a lower specific gravity and a lower magnetic force. However, it may make the toner have a low developing performance. The factor of a lowering of developing performance is that a low electrode effect results because the carrier becomes higher in electrical resistance. As the result, the toner at the rear end of a halftone area may come scraped off at the boundary between a halftone image area and a solid image area to make white lines, to cause image defects in which edges of solid image areas stand emphasized (hereinafter “blank areas”).
As a replacement for such a magnetic material dispersed resin carrier, Japanese Patent Laid-open Application No. 2006-337579 (Japanese Patent No. 4001606) also discloses a proposal of a resin-filled ferrite carrier having a void of from 10% to 60% and filled in its voids with a resin. Japanese Patent Laid-open Application No. 2007-57943 further discloses a proposal of a carrier the porous ferrite core material of which is filled in its voids with a resin and the structure of which has been specified.
In these proposals, porous ferrite cores are filled in their voids with a resin to make the magnetic carrier have a low specific gravity and a low magnetic force. Making the magnetic carrier have a low specific gravity and a low magnetic force brings an improvement in its durability and enables achievement of high image quality. However, it may make the toner have an inferior developing performance. The factor of a lowering of developing performance is that a low electrode effect results because the magnetic carrier becomes higher in electrical resistance. As the result, like the above, the toner at the rear end of a halftone area may come scraped off at the boundary between a halftone area and a solid area to make white lines, to cause image defects in which edges of solid areas stand emphasized (hereinafter “blank areas”). Also, in order to compensate any deficiency in developing performance, the Vpp (peak-to-peak voltage) of a development bias that is an alternating bias voltage may be set high, where the deficiency in developing performance can be compensated. In this case, however, a phenomenon of faulty images may occur in which ring-like or spot-like patterns appear on recording sheets. In addition, in general, upon flying of the toner from magnetic carrier particle surfaces in the process of development, electric charges having a polarity reverse to that of the toner are generated on the magnetic carrier particle surfaces. This is called counter charges. As the magnetic carrier becomes higher in electrical resistance, counter charges having come accumulated on the magnetic carrier particles become difficult to move to the developer carrying member side. Hence, any counter charges remaining on the magnetic carrier particle surfaces and the electric charges of the toner may attract each other to produce a large adhesion, so that the toner may become difficult to fly from the magnetic carrier particles, resulting in a low image density.
Thus, how to improve the stability and stress resistance of the two-component developer has been studied, but a two-component developer is long-awaited which can satisfy developing performance and running stability and can provide, over a long period of time, high-quality images free of any image defects.